In chromatographic detectors and numerous other applications, a window is required to pass light radiation between a chamber which is either pressurized or evacuated and an environment which is at atmospheric pressure or at some other pressure different from that of the chamber. This results in a relatively large pressure gradient across the window.
While windows for such applications may have curved faces and act as lenses to focus light radiation passing therethrough, typically, light radiation is to pass through the window undistorted. For this, it is necessary that the window have substantially flat surfaces which are oriented substantially perpendicular to the direction of the radiation. Typically, such windows are of the type shown in FIG. 1 where the window assembly 8 has a window 10 with a substantially rectangular cross section, the window being held, for example, between an outer sealing ring 12 and an inner sealing ring 14. Sealing rings 12 and 14 are held in place by, for example, a retaining ring 16 and are generally pressed between plates or other means utilized to precompress the sealing rings so that the seal is maintained when a pressure differential is applied across the window 10. This is necessary because, assuming pressure is applied to the window in the direction of arrow 18, window 10 moves slightly in response to the pressure, pressing against seal 12 while the pressure against seal 14 is reduced. Without precompression, seal 14 would fail, resulting in the potential loss of fluid or gas being retained to the right of window 10.
The configuration shown in FIG. 1 works reasonably well so long as the pressure differential across window 10 is not too great. However, for some applications, such as in a detector cell for super critical fluid chromatography (SFC), the pressure differential across the window may be 6,000 psi or more. In such applications, the window 10 is typically of a material such as quartz or fused silica which is transparent to ultraviolet light. Unfortunately, such materials have far greater resistance to compression stresses than to shear stresses. At high pressure, shear stresses may occur in such windows, causing potential failure thereof in at least two ways.
First, in order to prevent seal failure, it is necessary that the window assembly 8 have high precompression forces applied thereto. Theoretically, such forces would be provided uniformly around the perimeter of window 10, which window is typically round. However, it is difficult to avoid small irregularities in both the window 10 and in pressure plates utilized to apply the precompression forces. These small irregularities result in larger pressures being applied to some portions of the window perimeter than other portions and thus in shear stresses being applied to window 10. In applications where high precompression forces are required, such shear stresses can result in failure of window 10 during the precompression operation or thereafter.
Assuming the window 10 survives the precompression operation, when a pressure differential is applied across the window, the forces 18 are applied to the portion of window 10 inside sealing rings 12 and 14. These forces are countered by forces applied by sealing ring 12 to the outer periphery of the window. This results in substantial shear stresses being applied to the window which, at high pressure, can result in failure of the window.
Failure of the window 10 in a window assembly 8 being used in a piece of equipment such as a chromatographic detector can result in the loss of material being tested and, more important, result in expensive repairs and in the expense involved in what may be substantial equipment down time.
It is, therefore, desirable that windows and window seals be provided for high pressure applications which can withstand high precompression forces and high pressure differentials across the window without resulting in excessive shear stresses being applied to the window. In particular, it would be desirable if the window and seals could be designed such that most of the forces applied to the window would be compressive forces or stresses rather than shear stresses.